welcome to the garage Ted, check in at the BC bolters threads and you'll find some folks who might give you some 'hands on' help with the project, might even have some parts .... my suggestion is stick with the 3 speed, the granny 4 won't gain you a thing
a 4 speed in a 1/2T any length would be unusual for 58, an option [RPO318] .... takes a special driveshaft according to the assembly manual, and if your truck isn't set up for a steady bearing, it didn't have a 4 speed originally
Bill
It gains you a low gear for manuvering, I think the 2nd in the sm420 is a bit lower too, not so much clutch slipping etc. And I think 2nd is syncronized, and that is a gain.
